The present invention generally relates to deceleration control systems, and more particularly to a deceleration control system for an optical disk unit which moves a light beam from an optical head to a target track on an optical disk by controlling a velocity of a track actuator.
An optical disk unit has a large memory capacity because a track pitch of an optical disk can be set in the order of several microns. For this reason, the optical disk unit is suited for use in a computer system or the like as a memory device having a large memory capacity.
A track jump control of the optical disk unit is carried out in response to an external instruction which instructs a read access or a write access. When carrying out the track jump control, the velocity of a track actuator is controlled to move a light beam from an optical head to a specified target track on the optical disk. In other words, the velocity of the track actuator is controlled in a state where a tracking servo is OFF based on a velocity error between a target velocity and a beam moving velocity which is detected from a tracking error signal. In addition, an acceleration of the track actuator is controlled by applying an acceleration pulse voltage to the track actuator when starting the track jump, and a deceleration of the track actuator is controlled by applying a deceleration pulse voltage to the track actuator when ending the track jump.
However, the beam moving velocity before the deceleration control which is carried out immediately before the end of the track jump does not necessarily match the target velocity exactly. Normally, there is an error in the velocity control and the beam velocity does not become exactly zero even when the deceleration control is carried out for a predetermined time. For this reason, the beam position becomes unstable after the tracking servo becomes ON at the end of the track jump. Accordingly, it is desirable to realize a deceleration control system which decelerates the track actuator so that there is no velocity error when the tracking zero becomes ON at the end of the track jump.
FIG. 1 shows an essential part of an example of a conventional optical disk unit. An optical disk 10 is rotated at 3600 rpm, for example, by a spindle motor 24. An optical head 12 is arranged movable with respect to the optical disk 10, and a head driving motor 26 moves the optical head 12 in a radial direction of the optical disk 10. The optical head 12 emits a light beam which illuminates the optical disk 10 for reading information from and/or writing information on the optical disk 10.
A semiconductor laser 28 is provided as a light source within the optical head 12. A light beam emitted from the semiconductor laser 28 is directed to an objective lens 36 via a collimator lens 30, a deflection beam splitter 32 and a .lambda./4 plate 34, and the objective lens 36 converges the light beam into a beam spot on the optical disk 10. A reflected beam from the optical disk 10 is reflected in a perpendicular direction by the deflection beam spitter 32 and is supplied to a 4-segment photodetector 40 via a condenser lens 38.
In the optical disk unit described above, a plurality of tracks are formed on the optical disk 10 with a track pitch of 1.6 .mu.m, for example, along the radial direction of the optical disk 10. For this reason, the track position deviates greatly even by a slight eccentricity of the optical disk 10. In addition, the focal position of the beam spot deviates by an undulation of the optical disk 10. Therefore, the beam spot having a diameter of 1 .mu.m or less must follow such deviations in the track position and the focal position.
Hence, a focus actuator 42 is provided for adjusting the focal position by moving the objective lens 36 of the optical head 12 up and down in FIG. 1. In addition, a track actuator 44 is provided for controlling the tracking of the beam by moving the objective lens 36 in a direction which traverses the tracks on the optical disk 10.
The focus actuator 42 is controlled by a focus servo circuit 46. In other words, the focus servo circuit 46 drives the focus actuator 42 so that a focus error signal FES which is derived from an output light detection signal of the 4-segment photodetector 40 becomes a minimum.
The track actuator 44 is controlled by a tracking servo circuit 48 during a tracking servo operation in which the light beam is controlled to follow the target track. On the other hand, the track actuator 44 is controlled by a velocity control circuit 50 during a track jump operation in which the light beam jumps to an arbitrary track for making a new access.
FIGS. 2A and 2B are diagrams for explaining the conventional track jump control. FIG. 2A shows a case where the light beam jumps from an initial position S to a target track position TP. A velocity servo operation using a feedback loop is carried out to control the track actuator 44 so as to minimize a velocity error V.sub.e between a target velocity V.sub.t and a beam moving velocity V. At the same time, an acceleration pulse having an acceleration voltage +V.sub.a shown in FIG. 2B(A) is applied to the track actuator 44 for a predetermined time at the start of the track jump so that the light beam quickly reaches the target velocity V.sub.t. In addition, a deceleration pulse having a deceleration voltage -V.sub.a shown in FIG. 2B(B) is applied to the track actuator 44 for a predetermined time at the end of the track jump, so that the target track position TP is reached when the beam moving velocity V becomes zero and the tracking servo operation starts in this state. FIG. 2B(C) shows an actual beam moving velocity V.sub.L.
However, according to the conventional track jump control, the beam moving velocity V does not become exactly zero even when the deceleration control with respect to the track actuator 44 ends, and there is a problem in that the beam position becomes unstable after the tracking servo operation starts.
FIG. 3A shows a case where the beam moving velocity V at a time t1 when the deceleration starts is equal to the target velocity V.sub.t and no velocity error V.sub.e exists between the target velocity V.sub.t and the beam moving velocity V. In this case, the beam moving velocity V becomes exactly zero by the deceleration control which is carried out for a constant deceleration time T which is identical to the acceleration time. For this reason, the beam position is stable after the tracking servo operation starts.
On the other hand, FIG. 3B shows a case where the beam moving velocity V at the time t1 when the deceleration starts is greater than the target velocity V.sub.t and a velocity error +V.sub.e exists between the target velocity V.sub.t and the beam moving velocity V. Similarly, FIG. 3C shows a case where the beam moving velocity V at the time t1 when the deceleration starts is smaller than the target velocity V.sub.t and a velocity error -V.sub.e exists between the target velocity V.sub.t and the beam moving velocity V. In the cases shown in FIGS. 3B and 3C, the beam moving velocity V does not become exactly zero even when the deceleration control is carried out for the constant deceleration time T. As a result, the beam position becomes unstable after the tracking servo operation starts, and there is a problem in that a relatively long waiting time is required until the light beam reaches the correct read/write position.